Get rid of layers

They weren't being helpful (and in one case a bit wrong), and prevented
things from working on 5.6
Separate libdoc and bindoc checks

In some Linux distributions, perl is built with libdoc undefined, but bindoc defined.
The policy is to skip man3 pages and use perldoc for modules but to build man1 pages for end users.

Current (0.27) implementation of Module-Build-Tiny checks whether libdoc is defined,
but does not check bindoc.